The largest single application of lead-acid storage batteries is for the starting, lighting and ignition of automobiles, trucks and buses. These batteries are charged automatically from a generator driven by the engine while it is running and they supply power for the lights while the engine is shut down and for ignition and cranking when the engine is started. Lead-acid storage batteries are widely used in aircraft and boats with virtually unlimited applications also existing in non-motive situations.
Lead-acid batteries are formed from a series of lead-acid cells. A lead-acid cell consists essentially of positive plates containing positive active materials such as lead dioxide and negative plates containing negative active material such as sponge lead immersed in an electrolyte solution, typically dilute sulfuric acid. The respective positive or negative plates are connected in parallel. The power or current of the cell is determined by the number and size of the plates. The open circuit potential developed between each positive and negative plate is about 2 volts. Since the plates are connected in parallel, the combined potential for each cell will also be about 2 volts regardless of the number of plates utilized in the cell. One or more cells are then connected in series to provide a battery of desired voltage. Common low voltage batteries of 6 volts have 3 serially connected cells, 12 volt batteries include 6 serially connected cells and 24 volt batteries contain 12 serially connected cells.
The positive and negative plates are typically oriented vertically in a horizontal stacked relationship. As a result of this vertical orientation, electrolyte stratification commonly occurs vertically along the plate surfaces. This results in decreased battery performance. Some attempts have been made to prevent electrolyte stratification such as by stirring electrolyte by means of various mixing systems. These mixing systems are not only cumbersome but are expensive and subject to failure during the life of a particular battery.
Another problem with lead-acid batteries is their limited lifetime due to shedding of the active materials from the positive and negative plates. Pasted plate lead-acid batteries are by far the most common type of lead-acid battery. Typically, a paste of lead oxide is applied to the surface of the positive and negative grids. On application of electric potential, the lead oxide paste on the positive grid is oxidized to lead dioxide and the lead oxide of the negative grid is reduced to sponge lead. During operation these electrode materials shed and flake and fall down between the vertically oriented plates and accumulate at the bottom of the battery. After a period of operation sufficient flakes accumulate to short circuit the grids resulting in a dead battery cell and shortened battery life.
Lead-acid batteries are inherently heavy due to use of the heavy metal lead in constructing the plates. Modern attempts to produce lightweight lead-acid batteries, especially in the aircraft, electric car and vehicle fields, have placed their emphasis on producing thinner plates from lighter weight materials used in place of and in combination with lead. The thinner plates allow the use of more plates for a given volume, thus increasing the power density. Some of these attempts have included battery structures in which the plates are stacked in horizontal configurations. Higher voltages are provided in a bipolar battery including bipolar plates capable of through-plate conduction to serially connect electrodes or cells. The horizontal orientation of the grids prevents the accumulation of flake lead compounds at the battery bottom. Downward movement can be blocked by use of glass mat to contain the electrolyte. Also, stratification of electrolyte is confined and contained within the acid resistant glass mats by capillary action.
The bipolar plates must be impervious to electrolyte and be electrically conductive so that electrical current is conducted perpendicularly there-through to provide a serial connection. The bipolar plates also preferably provide a continuous surface to prevent sluffing off of active materials from the grids.
Most batteries utilizing bipolar plates have used metallic substrates such as lead or lead alloys. The use of lead alloys, such as antimony, gives strength to the substrate but causes increased corrosion and gassing. In addition to the problems of forming a liquid tight seal between the metallic substrate and adjacent nonconductive case (frame) materials, substrate corrosion, weight and strength factors have also been unacceptable. Furthermore, any attempt to reduce weight has lead to increased problems of strength and corrosion. Accordingly, a different approach must be used if acceptable weight and life are to be simultaneously achieved.
Alternate approaches have included plates formed by dispersing conductive particles or filaments such as carbon, graphite or metal in a resin binder such as polystyrene incorporating therein metal or graphite powder (U.S. Pat. No. 3,202,545), a plastic frame of polyvinyl chloride with openings carrying a battery active paste mixed with nonconductive fibers and short noncontacting lead fibers for strengthening the substrate (U.S. Pat. No. 3,466,193) a biplate having a layer of zinc and a polyisobutylene mixed with acetylene black and graphite particles for conductivity of the plate (U.S. Pat. No. 3,565,694), a substrate for a bipolar plate including polymeric material and vermicular expanded graphite (U.S. Pat. No. 3,573,122), a rigid polymer plastic frame having a grid entirely of lead filled with battery paste (U.S. Pat. No. 3,738,871), a plastic thin substrate having lead stripes on opposite faces, the lead stripes being interconnected through an opening in the substrate, and retained by plastic retention strips (U.S. Pat. No. 3,819,412) and a biplate having a substrate of thermoplastic material filled with finely divided vitreous carbon and a layer of lead antimony foil bonded to the substrate for adhering active materials (U.S. Pat. No. 4,098,967).
Some more recent examples of batteries containing bipolar plates are U.S. Pat. No. 4,275,130 in which the biplate construction comprised a thin composite of randomly oriented conductive graphite, carbon or metal fibers imbedded in a resin matrix with stripes of lead plated on opposite surfaces thereof. My copending application Ser. No. 279,841 filed July 2, 1981 entitled BIPOLAR SEPARATE CELL BATTERY FOR HIGH OR LOW VOLTAGE includes a biplate formed of a thin sheet of titanium is covered with a layer of epoxy resin containing graphite powder.
The resistance of such plates is always higher than predicted or desired due to the conduction path being formed of point to point contact of the dispersed conductive particles which are surrounded by highly insulative resin materials. The through-plate serial conductivity is limited and since the voltage of the cell is increased by including more bipolar plates; this increases the resistance of each cell and of the battery.